Several spaced-saving advantages are achieved by being able to fold the butt stock of a rifle relative to its receiver and barrel. For semiautomatic and automatic rifle deigns where the entire action, including the recoil spring/buffer system, is housed within the receiver, this merely requires providing a hinged attachment for the butt stock. However, in rifle designs, such as many AR-pattern firearms, where the bolt or the bolt carrier reciprocates into a receiver extension housed within or part of the butt stock where a recoil spring and/or buffer are located, this is more complicated.
Others have addressed this issue by providing a shortened receiver extension behind which the butt stock may be hinged to fold. Examples are shown in U.S. Pat. Nos. 7,966,761 and 9,347,738. This solution, however, fails to maximize the space-saving benefits of folding the butt stock. Others have used an adapter that provides a hinge with a fixed portion that is secured to the lower receiver and a pivoting portion that carries the buffer tube/butt stock assembly. Examples are shown in U.S. Pat. Nos. 8,769,855 and 8,991,088. Such adaptors add undesired weight and length to the extended butt stock.